1. Field of the Invention
This invention pertains in general to a method of synchronizing remote stations in a communications system and, more particularly, to a method for measuring delay between two remote stations in a communications system.
2. Description of the Related Art
Clock synchronization for stations at different localities with that of a central reference clock is required for many communications systems, such as synchronization of satellite clocks in a global positioning satellite (xe2x80x9cGPSxe2x80x9d) system, and synchronization of base stations and handsets in a Time Division Multiple Access (xe2x80x9cTDMAxe2x80x9d) or a Code Division Multiple Access (xe2x80x9cCDMAxe2x80x9d) cellular system. In the absence of an ideal clock, e.g., an atomic clock, synchronization is difficult to achieve without delay measurements.
For many cellular systems, such as the IS-95, CDMA2000, both developed by Qualcomm, Inc., and the Digital Enhanced Cordless Telephone (xe2x80x9cDECTxe2x80x9d) system of the European Telecommunication Standards Institute (xe2x80x9cETSIxe2x80x9d) standard, various methods have been proposed, including using the GPS system, making delay measurements through echo detection, and achieving synchronization using Integrated Services Digital Network (xe2x80x9cISDNxe2x80x9d) or High-Data-Rate Digital Subscriber Line (xe2x80x9cHDSLxe2x80x9d) interfaces aided by delay measurements to achieve base station synchronization. Each method has its merits and drawbacks.
GPS is accurate and stable but GPS signals cannot be detected indoors. Delay measurements through echo detection require a wire or a finite bandwidth for synchronization at the time of measurement. This measurement is not a simple peak detection task but rather a signal processing task requiring certain factors to be taken into consideration, such as severe distortion and noises. Furthermore, it is difficult to embed a delay measurement mechanism in an existing interface without having to redesign it. The third method of using synchronous data interfaces appears to be cost-effective because interfaces are already available in wireless communications systems. However, the algorithms implemented require a separate apparatus to measure an absolute delay.
In addition to base station synchronization, handset synchronization is also desirable for improving CDMA and TDMA systems. Base station and handset synchronization would also reduce the delay spread of handset frames at the base station. To achieve synchronization, delay measurement between the base station and the handset would be required. Synchronous CDMA systems are proposed to reduce interference and simplify implementation in a multi-user environment. Currently, uplink synchronous transmission scheme is an alternative technology in the third generation wide band CDMA standard.
The DECT system was designed for micro-cells, each cell having a base station. A synchronous DECT can also operate in large cells as may be required in a wireless local loop (xe2x80x9cWLLxe2x80x9d), where the handset to base station distance may be greater than 10 Km. However, according to interference calculations with the DECT design specifications, the system breaks down when frame synchronization from the handset is off by 11 microseconds. If the base stations are synchronized and the only source of timing error is due to air propagation delay difference between the nearest handset and the most-distant handset to the common base station, the largest cell radius is approximately 3 Km. In a 7-cell cluster, for example, there would require a central controller, and the maximum radius would be approximately 9 Km. This assumes the ideal case where the base station is only responsible for the handset in the cell. In a real fading environment where dynamic channel allocation prevails, the real radius of the DECT 7-cell cluster must be smaller than 9 Km because the delay spread may be as large as the diameter of the cluster. Therefore, handset synchronization is as important as base station synchronization in larger cells in a WLL using DECT or any other type of TDMA scheme.
Accordingly, the present invention is directed to a method for synchronizing two remote stations in a communications system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
To achieve these and other advantages, and in accordance with the purpose of the invention as embodied and broadly described, there is provided a method of measuring delay between a first station and a second station in a communications system, wherein the distance between the first station and the second station is more than negligible. The method includes transmitting a first signal from the first station to the second station and simultaneously resetting a counting clock in the first station, receiving a delayed first signal at the second station, transmitting the delayed first signal to the first station, receiving a further delayed first signal at the first station and simultaneously stopping the counting clock, calculating the number of counts elapsed in the counting clock between the resetting and the stopping of the counting clock, wherein the duration of each count of the counting clock equals the period of the counting clock, and multiplying the number of counts with the period of the counting clock to obtain the delay.
In one aspect of the invention, the simultaneously resetting a counting clock includes providing a first clock to reset the counting clock, wherein the first clock has a period equal to or greater than the delay between the first station and the second station.
In another aspect of the invention, the first station includes a first transceiver transmitting the first signal to the second station and providing the first clock signal to reset the counting clock.
In yet another aspect of the invention, the simultaneously stopping the counting clock comprises providing a second clock to stop the counting clock, wherein the second clock has a preferred period equal to the first clock.
As a preferred option the simultaneity in resetting and stopping is realized by using the rising or falling edge of input clock signals. Therefore, the preferred implementation of the signals is the rising or the falling edges of the clocks.
In still another aspect of the invention, the method further includes receiving at a third transceiver, coupled to a fourth transceiver in the second station, the delayed first signal from the first station and the fourth transceiver transmitting the delayed first signal to the first station.
Also in accordance with the present invention, there is provided a method of measuring delay between a first interface pair and a second interface pair in a communications system, wherein the first interface pair includes a first interface and a second interface and the second interface pair includes a third interface and a fourth interface, and wherein the distance between the first interface pair and the second interface pair is more than negligible. The method includes transmitting a first signal from the first interface and simultaneously providing a first clock signal to reset a counting clock, receiving a delayed first signal at the third interface, transmitting the delayed first signal from the fourth interface, wherein the distance between the third interface and the fourth interface is negligible, receiving a further delayed first signal at the second interface, and simultaneously stopping the counting clock, calculating the number of counts elapsed in the counting clock between the resetting and the stopping of the counting clock, wherein each count of the counting clock equals the period of the counting clock, and multiplying the number of counts with the period of the counting clock.
In one aspect of the invention, the method also includes applying delay locking to the first signal.
Further in accordance with the present invention, there is provided a method of synchronizing a handset to a base station in a wireless communications system that includes transmitting a first signal from the base station and simultaneously resetting a counting clock in the base station, receiving a delayed first signal at the handset, transmitting the delayed first signal from the handset, receiving a further delayed first signal at the base station and simultaneously stopping the counting clock, calculating the number of counts elapsed in the counting clock between the resetting and the stopping of the counting clock, wherein each count of the counting clock equals the period of the counting clock, and multiplying the number of counts with the period of the counting clock.
In one aspect of the invention, there includes repeating a predetermined number of repetitions of the calculating of the number of counts and adding the number of counts for the predetermined number of repetitions to obtain a sum, and dividing the sum by the predetermined number of repetitions to get the average count.
Further in accordance with the present invention, there is provided a method of synchronizing a first station with a second station in a communications system, wherein the distance between the first station and the second station is more than negligible that includes providing a first interface in the first station, the first interface transmitting a first signal and simultaneously providing a first clock signal to reset a counting clock in the first station, providing a second interface in the first station, providing a third interface in the second station, the third interface receiving a delayed first signal and transmitting the delayed first signal, providing a fourth interface coupled to the third interface in the second station, the fourth interface receiving the delayed first signal and transmitting the delayed first signal, wherein the distance between the third interface and the fourth interface is negligible, receiving a further delayed first signal at the second interface, and simultaneously stopping the counting clock, providing a counter to determine the number of counts elapsed in the counting clock between the resetting and the stopping of the counting clock, wherein each count of the counter equals the period of the counting clock, and multiplying the number of counts with the period of the counting clock.
In one aspect of the invention, the method also includes providing the first interface as a U-transceiver.
In another aspect of the invention, the method includes providing the first interface and the second interface as a master-slave pair.
Additionally in accordance with the present invention, there is provided a communications system that includes a first station transmitting a first signal and resetting a counting clock in the first station, a second station receiving a delayed first signal from the first station and transmitting the delayed first signal to the first station, the first station simultaneously stopping the counting clock upon receiving a further delayed first signal from the second station, wherein the distance between the first station and the second station is more than negligible, a counter coupled to the first station to determine the number of counts elapsed in the counting clock between the resetting and the stopping of the counting clock, wherein each count of the counter equals the period of the counting clock, and arithmetic means coupled to the counter for multiplying the number of counts with the period of the counting clock.
In one aspect of the invention, the first station is a base station.
In another aspect of the invention, the second station is a handset.
Still in accordance with the present invention, there is provided an apparatus for delay measurements that includes a first interface pair including a first interface and a second interface, the first interface transmitting a first signal and simultaneously resetting a counting clock, a second interface pair including a third interface and a fourth interface, the third interface receiving a delayed first signal from the first interface, and the fourth interface transmitting the delayed first signal to the second interface, wherein the distance between the third interface and the fourth interface is negligible, the second interface receiving a further delayed first signal and simultaneously stopping the counting clock, a counter coupled to the first interface pair to calculate the number of counts elapsed in the counting clock between the resetting and the stopping of the counting clock, wherein each count of the counting clock equals the period of the counting clock, and arithmetic means for multiplying the number of counts with the period of the counting clock.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structures and methods particularly pointed out in the written description and claims thereof, as well as the appended drawings.